In recent years, electronic apparatuses have become smaller and smaller in size, and therefore there has been a demand for higher-density electronic circuit boards for electronic apparatuses on which semiconductor devices or electronic devices are mounted. In order to satisfy such a demand, electronic circuit boards have been improved by, for example, reducing the pitch of wires to be soldered thereto. Meanwhile, from the viewpoint of environmental issues, there has been a demand for Pb-free solders that comply with, for example, the RoHS directive, and Sn—Ag—Cu-based eutectic alloys have already been practically used as solders for reflow soldering. On the other hand, as a high-temperature solder, Au-20Sn (melting point: 280° C.) is known and is hardly used because it is inferior to Pb—Sn-based solders in cost and mechanical properties. Therefore, other alloy systems which can be practically used as high-temperature solders have not yet been developed. Under the circumstances, high-temperature Pb solders are not yet included within the scope of the EU RoHS directive that restricts the use of hazardous substances such as Pb in electronic apparatuses and the like.
Further, in recent years, power ICs have been rapidly developed and have come to be practically used and the power capacity of power ICs has been increased steadily, and therefore various technologies to meet the demand for power ICs that can withstand high voltage and big current have been developed. Power ICs developed by new technologies are used under much higher voltage and current, and electricity conditions as compared to conventional power ICs, and are therefore often exposed to a current exceeding their rated current or a voltage exceeding their rated voltage transiently. This causes the problem of heat generation, which becomes increasingly prominent as output power increases. On the other hand, in order to ensure reliability for connection even under high power output conditions, there has been a demand for lead-free solders having a melting point higher than that of a conventional lead-free solder (e.g., Sn-3Ag-0.5Cu having a melting point of about 219° C.). However, high-temperature Pb-free solders suitable for practical use have not yet been developed. In addition, high-temperature Pb-free solders have a problem in that devices are degraded due to high soldering temperature and therefore their lifetimes are shortened.
Various soldering techniques effective for producing higher-density electronic apparatuses have been developed. For example, Patent Documents 1 and 2 disclose techniques for achieving higher-density packaging of electronic parts by reducing the pitch of wires and stacking a plurality of circuit boards.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2005-251889
[Patent Document 2] Japanese Patent Application Laid-Open No. 2001-267715